An image forming apparatus of an electrophotography type is provided with an optical scan mechanism including a rotating polygon mirror for deflecting a light beam from a light source with which a photosensitive body is irradiated. As a drive motor for driving the rotating polygon mirror to rotate, a brushless motor including a stator in which a plurality of coils is disposed and a rotor in which magnets are disposed may be used. For example, in an image forming apparatus, a plurality of Hall elements is disposed near the rotor of a brushless motor, where the timing of current-supplying to the coils is controlled by detecting the position and rotation speed of the rotor on the basis of output signals from the Hall elements. In another image forming apparatus, the timing of current-supplying to the coils is controlled by detecting the position and rotation speed of the rotor on the basis of voltages induced in the coils by the rotation of the rotor, without using the Hall elements.
Preferably, rotation control of a brushless motor involves proportional integral (PI) control so as to suppress a residual deviation between the rotation speed of the brushless motor and a target speed. However, in the technology for controlling the rotation of the brushless motor by detecting the position and rotation speed of the rotor on the basis of a change in magnetic flux produced in the coil, such as via the output signals from the Hall elements or the induced voltages in the above image forming apparatuses, the detected result of the rotation speed is greatly varied by the influence of noise and the like. Thus, if the well-known PI control is simply utilized in rotation control of the brushless motor, the rotation of the brushless motor may not be normally controlled because of an unwanted increase in integrated factors due to the fluctuation in rotation speed.
In the present specification, a technology for overcoming the inability to normally control the rotation of a brushless motor due to fluctuation in rotation speed caused by a change in magnetic flux of a coil is disclosed.